The invention relates to an M.times.N multiplex/demultiplex device suitable for single mode telecommunication systems using wavelength division multiplexed signals.
The principles of a Mach-Zehnder filter are described in patent application Ser. No. 08/052,523. In that application, wavelength filtering is accomplished by coupling light comprised of a plurality of wavelengths into N optical paths. The power is divided substantially equally among the N paths. Each optical path has a different length or propagates light of a given wavelength at a different speed. An alternative statement is, each path has a different optical path length for a given light wavelength. Because the optical path lengths are different, the light undergoes a different phase shift along each path.
The phase shifted light from each path is coupled into a single path where constructive and destructive interference occur. The optical path lengths are pre-selected to give constructive interference for particular light wavelengths and destructive interference for other light wavelengths. In this way, a pre-selected light wavelength is filtered out of the light signal. Note that the interference occurs when the light signals are coupled into a single path. That is, the coupling occurs at a spatial location, i.e., the location of the single output path. Hence the filtering depends upon both light frequency and the spatial relation of the filter components.
This principle of phase shifting by means of choosing a particular optical path length can also be applied to wavelength division multiplexing and demultiplexing. However, pre-selecting optical path lengths to give the proper phase shifts is more difficult. In multiplexing or demultiplexing, a plurality of wavelengths must be selected to constructively interfere, because information is being carried by each of a plurality of wavelengths.
In U.S. Pat. No. 5,234,672, Dragone, multiplexing or demultiplexing is accomplished using two star couplers connected by a set of optical waveguide paths denoted a grating. "The grating located between the two star couplers essentially consists of an array of curved waveguides of different length." (Col. 1, II. 42-44.) The difference in optical path length of light traversing the grating produces the interference which can result in multiplexing or demultiplexing. The problems encountered in making such a device include all those related to the design and manufacture of a star coupler. In addition, the grating paths must be arranged to limit phase error introduced by coupling between optical paths located close to each other. Further, the coupling of light between a planar waveguide, such as an output or input of a star coupler, and an optical waveguide fiber is a difficult, exacting task.
Thus there is a need for a coupling device which limits cross coupling phase errors among the optical paths.
In addition, an effective multiplexer or demultiplexer must satisfy a number of criteria, such as:
low attenuation; PA1 sufficient signal separation; PA1 environmental stability; PA1 mechanical stability; and, PA1 compatibility with an optical waveguide fiber system. PA1 for different wavelength light; PA1 for a different stress pattern in the waveguide; PA1 for different dopants or dopant amount in the waveguide; and, PA1 for different waveguide geometry, such as, core diameter PA1 photon induced refractive index change along all or part of the connecting fiber lengths; PA1 thermally induced refractive index change along all or part of the connecting fiber lengths; PA1 refractive index change induced by electro-optic interaction along all or part of the connecting fiber lengths; and, PA1 stress induced refractive index variation along all or part of the connecting fiber lengths, where the stress may be induced thermally, mechanically or by like means.
Because the multiplexer/demultiplexer includes at least one coupler, system compatibility includes coupler to waveguide fiber coupling. Thus, a multiplexer/demultiplexer device having a coupler comprised of waveguide fibers is more compatible with a waveguide fiber telecommunications system and is preferred.
Multiplexing is a cost effective method of increasing the capacity of installed fiber optic networks. Communications networks can be upgraded without disturbing or adding to the optical cable transmission system. As demand for telecommunications capacity increases, multiplexed systems have become more attractive. The search for low attenuation, reliable and, preferably, economical and easy to manufacture, multiplex and demultiplex devices has intensified.